1. Field Of The Invention
Gas generating compositions for inflating occupant restraint devices of over-the-road vehicles have been under development worldwide for many years and numerous patents have been granted thereon. Because of strict requirements relating to toxicity of the inflating gases, most gas generants now in use are based on inorganic azides, and especially sodium azide. One advantage of such known sodium azide gas generants is that the solid combustion products thereof generally produce a slag or "clinkers" which are easily filtered, resulting in a relatively clean gas. The ability of a gas generant to form a slag is a great advantage when the gases are used for inflation purposes, especially when the gases must be filtered as in the inflation of an automobile occupant restraint bag.
However, the use of the sodium azide, or other azides as a practical matter, results in extra expense and risk in gas generant manufacture due to the extreme toxicity of unfired azides. In addition, the potential hazard and disposal problem of unfired inflation devices must be considered. Thus, a nonazide gas generant exhibits a significant advantage over an azide-based gas generant because of such toxicity related concerns.
A fundamental problem that must be solved when using nonazide based gas generants is that it is easier to formulate slagging gas generants based on sodium azide than nonazide types because the combustion temperature is relatively low with azide-based gas generants. For example, the combustion temperature of a sodium azide/iron oxide slagging type generant is 969.degree. C. (1776.degree. F.) whereas, nonazide slagging type generants heretofore known have exhibited a combustion temperature of 1818.degree. C. (3304.degree. F.). Moreover, many common solid combustion products which might be expected from nonazide gas generants are liquids at the combustion temperature exhibited and are therefore difficult to filter out of the gas stream. For example, potassium carbonate melts at 891.degree. C. and sodium silicate melts at approximately 1100.degree. C.
The formation of solid combustion products which coalesce at high combustion temperatures, and at high gas flow rates, requires a special combination of materials. Early attempts at formulating nonazide gas generants resulted in semi-solid combustion products that were difficult to filter. It has been found that combustion products which are liquid at the combustion temperature must be cooled until solidified before filtering is successful because liquid products penetrate and clog the filter. It has also been found that cooling of the liquid combustion products results in cooling of the gas, which requires the use of more gas generant. A cooled gas is relatively less efficient for inflation purposes, especially with an aspirator system. The additional gas generant, in turn, requires more cooling and an additional filter as well as a larger combustion chamber.
Most azide-free, gas generant compositions provide a higher yield of gas (moles of gas per gram of gas generant) than conventional occupant restraint gas generants.
Although azide-free gas generating compositions offer numerous advantages over azide-based gas generants, it has been found difficult to produce gases which have sufficiently low levels of toxic substances. The toxic gases which are the most difficult to control are the oxides of nitrogen (NOx) and carbon monoxide (CO).
Most azide-free gas generants consist of carbon and nitrogen containing ingredients which, upon combustion, produce small, but undesirable levels of NOx and CO in addition to the desired products, nitrogen and carbon dioxide.
In combustion processes involving compounds containing both nitrogen and carbon it is possible to reduce or eliminate the CO by increasing the ratio of oxidizer to fuel. In this case, the extra oxygen oxidizes the CO to carbon dioxide. Unfortunately, however, this approach results in increased amounts of NOx.
The ratio of oxidizer to fuel may also be lowered to eliminate excess oxygen and provide a fuel rich condition which reduces the amount of NOx produced. This approach, however, results in increased amounts of CO.
Even though it is possible, by means of chemical equilibrium calculations, to find conditions of temperatures, pressure and gas generant composition which could reduce both NOx and CO to nontoxic levels it has been very difficult to accomplish this result in actual practice.
The aforesaid problems are solved by the present invention, which discloses several types of nonazide gas generants that yield solid combustion products which form a slag or clinkers at the relatively high combustion temperatures encountered with nonazide gas generants. The gas generants disclosed herein allow the use of simple, relatively inexpensive filters which cool the gas less and result in better pumping in an aspirated system. Taken together, these factors result in a simpler, less expensive and smaller airbag inflation system.
A problem solved by a preferred embodiment of this invention is that the NOx is controlled by means which are effective even though a limited amount of excess oxygen is present. This allows reduction of the CO level by the excess oxygen while, at the same time, lowering the NOx concentration to acceptable values.
2. Description of the Prior Art
An example of prior art teachings relating to the subject matter of the instant invention is found in European Patent No. 0,055,547 entitled, "Solid Compositions for Generating Nitrogen, The Generation of Nitrogen Therefrom and Inflation of Gas Bags Therewith". This patent describes use of alkali or alkaline earth metal salts of a hydrogen-free tetrazole compound and oxidizers of sodium nitrate, sodium nitrite and potassium nitrate or alkaline earth nitrates. A filter design is disclosed which utilizes fiberglass fabric that forms a tacky surface for particle entrapment. The filter has regions which cool and condense combustion solids. It is obvious from the disclosure and from the nature of the gas generating compositions that the solids produced do not form a slag and are difficult to filter.
European Patent No. 0,055,904 entitled, "Azide Free Compositions for Generating Nitrogen, The Generation of Nitrogen Therefrom and Inflation of Gas Bags Therewith" describes a filter used for particle entrapment. Oxidizers which contain no oxygen are used, and no mention of slag formation is made.
German Patent 2,004,620 teaches compositions of organic salts (aminoguanidine) of ditetrazole and azotetrazole that are oxidized using oxidizers such as barium nitrate or potassium nitrate. However, no compositions are mentioned which would lead to slag formation.
U.S. Pat. No. 3,947,300 entitled, "Fuel for Generation of Nontoxic Propellant Gases" discloses the use of alkali or alkaline earth metal azides that can be oxidized by practically any stable anhydrous oxidizing agent. The ratio of ingredients is selected to assure the formation of glass-like silicates with "as low a melting or softening point as possible" (column 2, lines 62-63 and column 4, lines 67-68). These silicates would be very difficult to filter in a high temperature system.
U.S. Pat. No. 4,376,002 entitled, "Multi-Ingredient Gas Generators" teaches the use of sodium azide and metal oxide (Fe.sub.2 O.sub.3). The metal oxide functions as an oxidizer converting sodium azide to sodium oxide and nitrogen as shown in the following equations: EQU 6 NaN.sub.3 +Fe.sub.2 O.sub.3 .fwdarw.3 Na.sub.2 O+2 Fe+9 N.sub.2
or EQU 4 NaN.sub.3 +Fe.sub.2 O.sub.3 .fwdarw.2 Na.sub.2 O+Fe+Feo+6 N.sub.2
The sodium oxide then reacts with the Feo forming sodium ferrite or with silicon dioxide (if present) to form sodium silicate or with aluminum oxide to form sodium aluminate, as shown below: EQU Na.sub.2 O+2 Feo.fwdarw.2 Na FeO.sub.2 (MP=1347.degree. C.) EQU Na.sub.2 O+SiO.sub.2 .fwdarw.Na.sub.2 SiO.sub.3 (MP=1088.degree. C.)
or EQU 2 Na.sub.2 O+SiO.sub.2 .fwdarw.Na.sub.4 SiO.sub.4 (MP=1018.degree. C.) EQU Na.sub.2 O+Al.sub.2 O.sub.3 .fwdarw.2 Na A10.sub.2 (MP=1650.degree. C.)
However, the above reaction products melt at temperatures well below the combustion temperature of compositions described in this invention and would, therefore, be difficult to filter.
U.S. Pat. No. 4,931,112 entitled, "Gas Generating Compositions Containing Nitrotriazalone" discloses the use of nitrotriazolone (NTO) in combination with nitrates and nitrites of alkali metals (except sodium) and the alkaline earth metals calcium, strontium or barium. However, the compositions taught in the patent are not capable of forming useful solid clinkers. For example, the two compositions given in Example 2 consist of different ratios of NTO and strontium nitrate which, upon combustion, would produce strontium oxide and strontium carbonate as fine dust since there is no low-temperature slag former present. Compositions claimed, utilizing mixtures of NTO and potassium nitrate, likewise will not form a useful solid clinker since potassium carbonate would be produced which would be a liquid at the combustion temperature and no high temperature slag former is present. The hydroxides mentioned are very unlikely to be formed because the excess carbon dioxide would convert the metal oxides to carbonates in preference to hydroxides. Even if some hydroxides were formed they would be the wrong type of slag former to promote clinker formation.
U.S. Pat. No. 4,909,549 entitled, "Composition and Process for Inflating a Safety Crash Bag" discloses the use of alkali metal salts, alkaline earth metal salts or ammonium salt of a hydrogen containing tetrazole in the range of about 20 to about 65 wt. %. The effectiveness of alkali metal compounds, at these or lower concentrations, was not known.